The invention relates to a fan wheel designed as a radial or diagonal fan, comprising a top plate with an inlet port, a base plate, and a plurality of fan blades distributed around the inlet port and around an axis of rotation, as well as blade ducts formed respectively between the adjacent fan blades in a circumferential direction, said fan blades leading radially or diagonally outward from the area of the inlet port and forming blow-out ports in the external region, the blade ducts being designed, with respect to their effective flow cross-section, as large enough that during operation, a turbulent flow with a Reynolds number markedly greater than 2300 is achieved, the top plate and/or the base plate displaying a rotationally asymmetrical geometry.
Fan wheels of his kind are termed turbo-machines (turbo-fans). They are characterized by the very high Reynolds number Re, which, with a value of at least 5000 (i.e. Re≧5000) is significantly greater by a factor of >2 than the sufficiently well known threshold value of approximately 2300 between laminar flow (Re<2300) and turbulent flow (Re>2300). In most cases, however, Re is actually ≧10000 (factor>4) and can go up to several 10000 (for example 35000). Due to the turbulent flow in the blade ducts, high efficiency is achieved in the region above 0.6, and up to at least 0.8 (60-80%). It can be presumed that that the flow through the flow ducts is predominantly a so-called tubular flow so that the Reynolds number Re can be calculated based on the characteristic quantities for a tubular flow, i.e. the flow width, which is typically an idealized substitute inner diameter d, the value of the flow velocity vm averaged over the cross section, and the (kinetic) viscosity ν of the respective medium. The dimensionless Reynolds number is then:
  Re  =                    v        m            ·      d        v  
In the equation above, it is assumed that air has a kinetic viscosity of v=1.5·10−5 m2/s.
Efficiency is defined as the ratio of utilized output power to supplied input power. For an electric motor drive, the electric input power or a mechanical shaft drive power used to rotate the fan can be applied as input power. In this case, the so-called “free-flowing efficiency” ηff is defined as:
      η    ff    =                              V          .                ·        Δ            ⁢                          ⁢              p        ff                    P      w      
ηff is the ratio of the product of volume flow {dot over (V)} multiplied by pressure difference Δρff to input power Pw. The relevant values are measured according to ISO 5801.
Furthermore, in connection with the invention, the term “rotationally asymmetrical” means that any two different radial cross sections through the base plate and/or the top plate in two planes that contain the rotational axis and include a specific differential angle in the circumferential direction are not congruent when there are different circumferential angles, but rather deviate from one another. In this case, a deviation could in principle be present in the direction of the axis of rotation (axially) and/or in the radial direction (radially). In other words, this means that in the case of a rotational asymmetry, a rotation of the body through specific angles around the axis of rotation does not map the object or its sectional plane on itself.
A fan wheel is described in various versions in the publication JP 2001-263 294. In this case, the top plate or the base plate, or each of the two, has a contour that is stepped obliquely in the circumferential direction. This step shape, which is oblique in the direction of rotation, is meant to reduce a tendency of the airflow to break away, and in this way to have a positive influence on noise and efficiency. The step shape results in each fan blade having different outlet widths (measured axially) on its suction side and its pressure side, which means, depending on the embodiment, that the outlet width on the suction side can be smaller or greater than the outlet width on the pressure side.
EP 1 933 039 A1 describes a radial fan with ribs, recesses or as the case may be, indentations on the outside of the top plate. This configuration is intended to reduce noise as a result of specific flow routing.
The additional publication, EP 1 032 766 B1, describes a fan wheel, in particular, as a turbocharger. In this fan wheel, blades are formed by embossings on at least one of the two plates (base plate and/or top plate). These embossings also produce a rotationally asymmetrical geometry. However, this publication is not concerned with exerting an influence on flow; it is chiefly concerned with aspects of the manufacturing process and the factors that promote stability.
A rotationally asymmetrical geometry is also produced according to the publication DE 32 47 453 C1, by means of cupping. Blade parts herein are molded from a base plate and an annular disk opposite to it after heating said blade parts, and are then fitted together to form a fan impeller by welding together the respective crest sections of the blade parts. However, as in the case of EP 1032 766 B1 cited above, this publication is not concerned with influencing flow. Its sole purpose is to simplify the production of a fan impeller from thermoplastic plastic and to increase the impeller's stability.
The publication US 2007/01 16561 A1, or as the case may be the corresponding U.S. Pat. No. 7,455,504 B2 describes different embodiments of a quite special flow machine that is intended, in very small embodiments, for use in computers. Here, the flow ducts are designed with a very small flow cross-section in order to achieve a laminar flow. Consequently, this is not a “turbo-machine” in the sense of the invention; because in that prior art, the specific intention is that the Reynolds number be less than 2300. In concrete terms, the entire flow cross-section is divided into a plurality of small flow ducts. This is achieved, for example, by means of a honeycomb-like structure, which also appears to produce a rotationally asymmetrical design. However, this is done only to avoid forming any flow ducts, in order to ensure laminar flows. Features of these known embodiments cannot be applied to a “turbo-machine” of the type described in the present invention, because they involve completely different operating principles. For example, the peak efficiency of the known “laminar machine” is only around 0.2 (20%).
Numerous further publications describe rotationally symmetrical fan wheels. The following publications can be mentioned solely as examples: DE29 40 773 C2, DE199 18 085 A1, EP 1 574 716 B1, and DE203 03 443 U1, as well as GB 438 036A1. Such fans, with rotationally symmetrically designed base plates and/or top plates display in part, both in the direction of the axis of rotation and in a circumferential direction, highly irregular velocity and pressure distributions, i.e. locally elevated velocity/pressure ranges. This can result in flow breakaway and even backflow, which in turn cause aerodynamic losses, efficiency losses, and increased noise emission. Regarding the cited document GB 438 036 A, it should also be mentioned that each fan blade and/or each top plate is meant to be comprised of two separate layers that are connected in a way similar to corrugated cardboard via wavy connecting webs. This results in a rotationally asymmetrical profile between the two layers, however the surfaces of the top plates, which are responsible for the flow properties, are nevertheless rotationally symmetrical. There is no flow through the hollow space between the layers which is reinforced with “corrugated cardboard.”